An adhesive that has the ability to establish multiple discrete electrical connections, often in extremely close proximity, between two components is commonly referred to as an "anisotropically conductive adhesive" or as a "z-axis adhesive." A typical use for this type of material is to provide connection between a flexible printed circuit and a flat panel display.
Several performance and handling characteristics are required of a z-axis adhesive. With regard to performance, the ability to establish and maintain high integrity electrical connections is the most critical need. Generally, this means that the adhesive should possess moderately high elastic modulus and good creep resistance, even at elevated temperatures and humidities. In addition, good adhesion to both substrates being connected is essential not only to provide good working strength during product assembly but also to prevent delaminations during exposure to extreme environments which can lead to total failure of the connections.
With regard to handling characteristics, the severity of the bonding process as defined by required bonding time, adhesive temperature and bond pressure is the most important. In general, the processing temperature must exceed the highest temperature for which good interconnection integrity is required. However, in the interest of avoiding the potential for damage to either the substrates or any adjacent components it is desirable to develop materials which can minimize the gap between maximum test temperature and processing temperature. This concern becomes particularly important in applications calling for the use of flexible printed circuits using base films other than polyimide. For a growing number of applications, substrates are being developed which can't tolerate bonding temperatures higher than 120.degree.-130.degree. C. Despite this constraint, reliability testing needs cannot be relaxed. Therefore there is a need for materials that can be bonded at temperatures only slightly higher than the maximum test temperature.
Some of the first z-axis adhesives to be developed utilized simple, non-reactive hot-melt type compositions such as styrene-butadiene-styrene block copolymers. These materials offered such conveniences as indefinite shelf life, rapid processing and easy reworkability. However, a considerable drawback to these materials was their somewhat poor resistance to elevated temperature and humidity aging. The required bonding temperature of 150.degree. C. for these materials was sufficiently modest for some but not all applications, and the interconnection stability for most applications was marginal at 125.degree. C.
As the applications of z-axis adhesives evolved the need for reliability increased. Hence it became necessary to develop cross-linkable adhesive compositions in order to provide improved heat resistance. The challenge became to provide curable compositions that didn't compromise the convenient handling features of the hot-melt materials. An example of this type of second generation material was described in JP1-113479, which taught an epoxy-based composition which utilizes an epoxy modified imidazole accelerator and a thermoplastic component. This type of formulation was claimed to exhibit up to 6 months shelf-life at room temperature, but required a bonding temperature of at least 170.degree. C. Another similar example of this type of material was described in U.S. Pat. No. 5,001,542, which described the use of a microencapsulated accelerator. Once again, a modified imidazole requiring bonding temperature of at least 170.degree. C. was used as the accelerator. Both of the above examples rely on the accelerator to catalyze an epoxy--epoxy homopolymerization. Cyanate ester based compositions were described in U.S. Pat. No. 5,143,785 for the same type of material, but these materials also require cure at greater than 170.degree. C.